Optical systems provide extremely fast and effective means for processing information. In a typical system, an image comprising data is modulated into a coherent light beam. This can be performed by a spatial light modulator placed in the beam. The resulting spatially modulated beam then enters a series of optical elements which filter and process the image, and a detector records the final output. The list of applications for these systems is long, including image and data processing, pattern recognition, optical computation, and high density data storage systems such as holographic data storage systems.
Prior art holographic data storage systems have primarily made use of Fourier hologram recordings. In such a system, a collimated laser beam is directed through a spatial light modulator (SLM) which impresses into the beam the desired optical data to be stored in the system. The spatially modulated output of the SLM is directed towards a positive lens. The SLM is located at a front focal plane of the lens, while a holographic storage material, commonly a photorefractive crystal or photosensitive material, is located at a back focal plane. It is well known that after passing through the lens and arriving at the crystal, the modulated beam generates the spatial Fourier transform of the original data (see, for example, J. W. Goodman, Introduction to Fourier Optics, McGraw-Hill, 1968). Hence, a volume hologram is formed in the crystal by the interference of the modulated beam with a reference laser beam directed into the crystal.
Once the hologram is created, the original signal can be retrieved by directing the reference beam into the crystal. However, the reconstructed beam initially contains the transformed data not the original data. To render the optical data in its original form produced by the SLM, the reconstructed beam must be focused by a lens, referred to hereafter as a readout lens. Generally, the readout lens focuses the beam on the surface of a spatial light detector, most commonly a charge coupled device (CCD). The resulting image is that of the original data and is consequently recovered by the detector.
A 4-focal length (4-f) Fourier holography arrangement has traditionally been used for holograhic data storage. In a 4-f system, a spatial light modulator is placed at the front focal plane of a first lens and the holographic storage medium is placed at the back focal plane (the Fourier plane) of the first lens. A second lens is placed after the medium at a distance from the first lens equal to the sum of the focal lengths of the first and second lens, and a detector array is placed at the back focal plane of the second lens. Each pixel imaged on the detector array is recorded throughout the medium. The device is therefore less susceptible to error than a device which records data only at an image plane. Also, the tolerances of lens placement and design are less strict. However, a 4-f system has the disadvantage of requiring a relatively large number of optical components, which increases the cost of such systems.
Prior art holographic data storage systems generally require a relatively large number of complex and costly optical components.